As steel pipe for line pipe used for the trunk lines of pipelines important as a long distance transport method of crude oil, natural gas, etc., high strength, high toughness steel pipe for line pipe has been proposed (for example, Japanese Unexamined Patent Publication No. 62-4826A). Up to now, commercialization of high strength steel pipe of up to X80 in the American Petroleum Institute (API) standards has been pushed. In recent years, however, further higher strength line pipe has been demanded due to (1) the improvement in transport efficiency due to higher pressures and (2) the improvement in on-site installation efficiency due to the reduction of the outside diameter and weight of line pipe.
For example, if using X120 grade line pipe having a 900 MPa or higher tensile strength, it is possible to increase the internal pressure, that is, the pressure of the crude oil or natural gas, to about double that of 65 grade line pipe, so about double the amount of crude oil or natural gas can be transported. Further, if raising the strength of the line pipe to improve the resistance to internal pressure, compared with making the thickness greater, it becomes possible to cut material costs, transport costs, and on-site welding and installation costs and possible to greatly save on pipeline laying costs.
Further, pipelines are often laid in artic regions, so have to be superior in low temperature toughness. Furthermore, at the time of installation, the ends of the line pipes are connected, so superior on-site weldability is also demanded. To satisfy this demand, steel pipe for high strength line pipe with a base metal of a microstructure mainly composed of a mixed structure of bainite and martensite suitable for X120 grade line pipe higher in strength than the steel pipe for line pipe proposed in Japanese Unexamined Patent Publication No. 62-4826A has been proposed (for example, Japanese Unexamined Patent Publication No. 10-298707A, Japanese Unexamined Patent Publication No. 2001-303191A, and Japanese Unexamined Patent Publication No. 2004-52104A).
Furthermore, when producing steel pipe, steel plate is shaped into a tube and the seam portions are seam welded. When toughness and productivity are demanded as with steel pipe for high strength line pipe, submerged arc welding from the inner surface and outer surface is preferable for the seam welding. When welding a steel material a plurality of times in this way, the heat affected zone (HAZ) coarsened by the input heat from the earlier welding is reheated by the input heat of the later welding and the toughness drops.
It is known that the drop in toughness of this reheated HAZ (reheated HAZ) is due to the formation of a mixture of martensite and austenite (MA). To solve this problem, some of the inventors proposed the method of suppressing the drop in toughness by reducing the area ratio of the MA of the reheated HAZ and suppressing hardening of the reheated HAZ (for example, Japanese Unexamined Patent Publication No. 2004-68055A and Japanese Unexamined Patent Publication No. 2004-99930A).
However, the method proposed in Japanese Unexamined Patent Publication No. 2004-68055A heat treats the weld zone of the steel pipe. For this reason, a method not requiring heat treatment of the weld zone and, in the case of heat treatment, technology for improving the toughness at a low temperature is being demanded. Further, the method proposed in Japanese Unexamined Patent Publication No. 2004-99930A requires control of the cooling rate after welding. Depending on the production conditions, it is sometimes difficult to limit the cooling rate of the weld zone. For this reason, technology for improving the reheated HAZ toughness without relying on the cooling rate of the weld zone is also being demanded.